1. Field of the Invention
The present invention relates to a focus detection apparatus of a camera, which detects the focus condition of a photo-taking lens of the camera, and more particularly to a focus detection apparatus having larger focus detection field, where the focusing condition can be detected, than the conventional focus detection apparatus.
2. Description of the Prior Art
It is necessary to expand the focus detection field so that focus detecting operation may be possible with respect to the object which is moving or is in low contrast condition.
Some one-area focus detection apparatus, which have characteristic re-imaging lens and a lengthened CCD (charge-coupled device) line sensor in order to expand a focus detection area, are known. For example, in Japanese Laid-Open publication No. 156609/1987, the re-imaging lens has a concave surface in its photo-taking lens side, and in United patent application Ser. No. 4,808,808, a prism is arranged between the re-imaging lens and the CCD line sensor. It is possible for the above-mentioned focus detection apparatus that the focus detection area is expanded without regard to an open aperture value of the photo-taking lens.
However, in the above-mentioned apparatus, light fluxes used for detecting the focus condition are refracted suddenly by the concave surface of the re-forming lens or the prism arranged between the re-imaging lens and the CCD line sensor. Therefor, secondary images formed on the CCD line sensor by the re-imaging lens tend to have various aberrations, and it is necessary in order to remove the aberrations that the minute re-imaging lens or the minute prism is manufactured precisely. As a result of this, the production cost thereof becomes very expensive.
Another well-known multi-area focus detection apparatus, which detect focus conditions with respect to the axial and off-axial focus detection areas, is shown in FIGS. 1 and 2. In this apparatus, a pair of light fluxes used for detecting the focus condition with respect to the axial focus detection area FA0, containing a pair of principal rays 11 and 12, are focused on a CCD line sensor P0, through an opening E0 provided on a field mask FM, a condenser lens L0, a pair of aperture openings Al and A2 provided on an aperture mask AM, and a pair of re-imaging lenses L1 and L2 formed on a re-imaging lens plate L. And a pair of secondary images are formed on the CCD line sensor P0. On the other hand, a pair of light fluxes used for detecting the focus condition with respect to an off-axial focus detection areas FA01 (FA02), containing a pairs of principal rays 13 and 14 (15 and 16), are focused on a CCD line sensor P01 (P02), through an opening E01 (E02), condenser lens L01 (L02), an aperture openings A3 and A4 (A5 and A6), and a pair of re-imaging lenses L3 and L4 (L5 and L6). And a pair of secondary images are formed on the CCD line sensor P01 (P02). The focus condition of the photo-taking lens TL can be detected by means of detecting three pairs of the secondary images.
FIG. 1 shows a view finder image corresponding to the above mentioned focus detection apparatus. As described above, the focus detection areas FA0, FA1 and FA2 correspond to the CCD line sensors P0, P01 and P02, respectively. As shown by dotted lines in FIG. 1, there are two spaces which have no focus detection area among the focus detection areas FA0, FA01 and FA02. Hereinafter, the space is referred to as "the non-detection space". If there is the non-detection space among the focus detection areas, it is impossible to detect the focus condition with respect to an object existing in the non-detection space. Especially, when the object is moving, it may often come in the non-detection space.